1. Field of the Invention
The present invention relates to a vulcanizing mold for vulcanizing and molding a rubber molded article such as a tire and to a cleaning method for the same.
2. Description of the Prior Art
To form a tire, a vulcanizing mold (to be referred to as “mold” hereinafter) is used in which pressure is applied to the inside of the molded green tire to pressure contact the outer surface of the above green tire to the inner wall of the heated mold in order to vulcanize the crude rubber by heat and pressure. Exhaust passages such as micro-vents, slits, vent holes or cross vents are formed in this mold to remove air contained between the outer surface of the tire and the mold and gas generated from the green tire. However, when the above micro-vents, slits, vent holes or cross vents are used, a rubber material enters the holes of the vents during molding and a large number of projections called “spew” are formed on the surface of the vulcanized tire, thereby impairing the appearance of the tire. Therefore, trimming is indispensable. Even when trimming is carried out, the height of trimming may vary or part of the tire may be untrimmed, which is not preferred from the viewpoint of the appearance of the tire. In addition, a trimming apparatus is required, thereby boosting cost. Since the occlusion of vent holes caused by the slivering of spew occurs in the mold, the spew must be removed by using a tool such as a drill. However, as there are hundreds of vent holes in the mold, the work of removing the spew takes a lot of time and labor. In the case of cross vents, the spew cannot be removed by using a tool or the like.
A film-like projection called “lip” is formed in the case of slits, which is not preferred from the viewpoint of the appearance of the tire. When a large lip crossing the main groove of the tire is formed, it may dam the main groove having a drainage function to prevent drainage, thereby causing a problem with safety driving, such as triggering of hydro-planing while driving on a wet road.
To cope with the above problem, a mold having porous air vents, each comprising a porous member having a large number of pores for transmitting only gas formed in a substrate such as a metal substrate or a porous member made from a porous material having a large number of pores such as a sintered metal, has been developed as a mold which does not require the above trimming step.
FIGS. 9(A) to (C) show an example of a vulcanizing mold 10P for rubber molded articles having porous members of the prior art. This vulcanizing mold 10P for rubber molded articles is made by putting together a plurality of segments 11 into a substantially cylindrical form, and an opening 12 into which a rubber material (bladder) for supplying a green tire to be vulcanized, heat and pressure is inserted is formed in the core portion of the above mold 10P. Each of the above segments 11 consists of an outer case 51 made from a steel material and a plurality of aluminum pieces 52 secured to the inner wall of the outer case 51 by unshown bolts. The surfaces (portions exposed to the inner wall of the mold 10P) of the above pieces 52 are tire forming surfaces, that is, portions to which the rubber of the green tire to be vulcanized is contacted.
A plurality of holes 52S which communicate with exhaust holes to be described hereinafter formed in the outer case 51 are formed in each of the above pieces 52. A plurality of vent pieces (to be referred to as “porous air vent” hereinafter) having a large number of straight pores 53S with a diameter of 10 to 40 μm and communicating with the exhaust holes 51S of the outer case 51 are fitted in the respective holes 52S as shown in FIGS. 10(A) and 10(B). In FIG. 10(B), an upper side (piece 52 side) of the figure is a rubber side and a lower side (outer case 51 side) of the figure is a mold side. Thereby, air between the mold 10P and the outer surface of the tire and gas generated from the green tire during vulcanization are absorbed from the above pores 53S which are exhaust passages by unshown exhaust means such as a vacuum pump connected to the above exhaust holes 51S and discharged to the outside of the mold 10P from the exhaust holes 51S of the outer case 51 through a gap 54 between the pieces 52 and the outer case 51.
However, in the porous air vent 53 of the prior art, as shown in FIG. 11, since the diameters of the above pores 53S are fixed from the cavity side to the opposite side (outer case 51 side), when the occlusion of the pores 53S by plug materials p such as oil, chemicals and metal fumes discharged from the green tire during vulcanization and fine particles of a polymer and carbon occurs on the cavity side, it is difficult to discharge the above plug materials p to the exhaust side (outside of the mold 10).
When the above plug materials p adhere to the cavity side of each pore 53S like tar, the passage of the air vent is narrowed and the plug materials adhere to the pores 53S more easily and form a bridge to occlude the pores 53S, thereby greatly reducing exhaust efficiency and causing a tire failure in an early stage. Therefore, the above mold 10P must be cleaned frequently and the operation efficiency of the mold lop is low. Once the porous member is plugged, it is extremely difficult to remove the plug materials with a drill or the like. Particularly when the porous member is a sintered metal, as each opening constituting a ventilation passage is not straight, a special cleaning method must be used and it is difficult to clean the mold. When a sintered metal is used as the porous member, the meshed patch of each vent is formed on the surface of a tire, impairing the appearance of the tire.
When vulcanization is repeated, plug materials such as a rubber material and a polymer produced by the slivering of spew and fine particles of carbon adhered during vulcanization are firmly adhered to the surface of the mold, thereby staining the surface of the mold. To remove these stains on the surface of the mold, the following methods have been employed.    (1) physical methods for blasting a shot of plastic beads or glass beads, or dry ice, affixing cleaning rubber, or applying a laser beam or a jet of high-pressure water (so-called peeling; peeling of adhered products)    (2) physico-chemical methods making use of plasma, ultra-criticality to pseudo-criticality, and electrolysis    (3) chemical methods for immersing a mold in an amine-based cleaning liquid (+), oxygen-based cleaning liquid or sodium bicarbonate-based cleaning liquid to expand stains and removing them by high-pressure water.
However, in the methods (1), only stains on the surface of the mold can be removed but stains in the exhaust passages cannot be removed and the surface of the mold may be damaged. In the methods (2), the electric erosion/melting or deformation/modification of the surface of the mold easily occurs.
In the methods (3), it takes time because the mold is cleaned only with a cleaning liquid, the cleaning liquid is apt to deteriorate, and chemical treatment equipment is needed. Therefore, the methods have economical and spatial problems.
It is an object of the present invention which has been made in view of the above problems of the prior art to provide a vulcanizing mold for rubber molded articles which suppresses the occurrence of the occlusion of exhaust passages with plug materials and makes cleaning easy as well as a method of cleaning the mold.